WIP: Perform blocking metadata and link checks on changed files

We want to ensure consistency and validity of the metadata and external
URLs provided in documentation. We currently have warnings in place for
broken external URLs, however as these checks are performed across the
entire site and only a warning (as it's not reasonable for someone to
fix up entirely unrelated links to get an MR accepted), there isn't
really a mechanism in place to ensure some targetted improvement as the
site is changed.

To remedy this, determine which files are added/modified in a given MR
and ensure that the sumbitter has corrected any broken links in the
documentation that they've touched and are providing the required
metadata.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>